1. Technical Field
This disclosure relates to wireless energy transfer and control thereof.
2. Description of Related Art
There is growing interest to transfer power wirelessly in order to charge or power various consumer electronic devices ranging from cell phones to heavy duty industrial equipment such as crane motors. In this regard, inductive charging uses an electromagnetic field to transfer energy between two objects. Energy is sent through inductive coupling to an electrical device used to either power or charge the device. The removal of a direct (i.e., wired) electrical contact between a transmitter and receiver circuit provides many advantages.
For example, inductive charging may lower the risk of electrical shock, when compared with conductive charging, because there are no exposed conductors. Further, it provides the ability to hermetically seal the power transmitter circuit and the receiver circuit. Such encapsulation of circuitry offers uses where water impermeability is desired, such as charging implanted medical devices or charging personal hygiene devices such as toothbrushes and shavers, which may come into contact with water.
Even in applications where electrical shock or hygiene may not be of paramount concern, inductive power transfer is often desirable. For example, the connection between a device to be powered and its power source may become frail over time. Further, establishing physical contact between a charger and the device are relatively time consuming.
Accordingly, wireless power transfer increases safety, reliability, and time efficiency, while decreasing maintenance cost.
FIG. 1 illustrates a typical wireless power transfer system 100 which includes a transmitter circuit 104 (i.e., primary unit) and a receiver circuit 110 (i.e., secondary unit). A transmit coil (TX) 106 is energized to transmit a time varying magnetic field. The magnetic field generated by the transmit coil 106 induces current to flow in a receive (RX) coil 108 in the vicinity of the generated magnetic field. In a resonant system 100, this ac current flows back and forth between the receive coil 110 and a tank capacitor (not shown) in the receiver circuit generating an output voltage. This ac voltage can then be rectified to produce a dc voltage at the output across a load RL 112. This regulated output voltage at the output of the receiver circuit 110 is capable of delivering dc current to power the load RL 112 on the receiver side.
The secondary unit 110 may use regulation to control the amount of power delivered to the load 112. In one example, this regulator can be a DC/DC converter that regulates its output voltage. For a fixed resistive load, the regulated output voltage controls the amount of current in the load 112 and hence the total amount of power delivered to the resistor 112.
Notably, there is no direct feedback from the receiver circuit 110 to the transmitter circuit. Without such feedback, the transmit circuit on the primary unit 104 may energize the transmit coil at a maximum level to accommodate a worst case operating condition on the secondary unit 110. For example, the worst case operating condition may involve maximum load current and lowest coupling between the two coils (i.e., coils 106 and 108). Energizing the transmit coil 106 at the maximum level may cause large power dissipation in the transmitter circuit 104 as well as the transmit coil 106 due to non idealities that exist in a real transmitter circuit 104 and a real transmit coil 106.
In this regard, the transmitter circuit 104 and the transmit coil 106 dissipate power even when there is no receiver circuit 110 or an RL 112 present. The non idealities (e.g., dissipative elements in the transmitter circuit 104) include switching losses. Further, the losses in the transmit coil 106 include I2R losses. One significant non ideality may be the parasitic series resistance of the transmit coil 106 itself. Such non-idealities are energy inefficient.
Accordingly, energizing the transmit coil 106 at a maximum level is not desirable if the receiver circuit 110 does not spend a substantial part of its operation delivering maximum power to the load RL 112. For example if the load 112 is the battery of a cell phone, the battery only needs to be charged at maximum current during a short period at the beginning of each charge cycle. Continuing to energize the transmit coil 106 at maximum level after this short period is energy inefficient.
In one approach to optimizing the overall efficiency of power transmission from the primary 106 to the secondary 110 side, physical feedback from the receiver circuit 110 to the transmitter circuit 106 is used. In this regard, there are many methods to provide such feedback.
For example, there may be an opto-coupler to provide direct feedback information from the receiver circuit 110 to the transmitter circuit 104. In another example there may be a dedicated radio link with a wireless communication protocol to provide this feedback information. Still other methods may employ indirect observation of primary unit ac waveforms by slight modulation of the secondary resonance capacitor. This modulation can then be used as a low bandwidth radio link to send the feedback information from the receiver circuit 110 to the transmitter circuit 104.
However, the prior art essentially relies on direct feedback information from the receiver circuit 110 to the transmitter circuit 104 in order to regulate the power transfer from the transmitter circuit 104 to the receiver circuit 110.
Accordingly, it would be desirable to provide active control of the power transfer from the transmitter circuit to the receiver circuit and the load by indirectly determining the conditions of the receiver circuit and the load.